Philotheus was a writer of note, writing works on the theology of the Uncreated Energies of God and attacking the scholastic philosophy that was then current in the Western church. His most famous work is the Hagiorite Tome, the manifesto of the Mount Athos monks on how the Saints partake of the Divine and uncreated Light that the Apostles beheld at the Transfiguration of Jesus.

In 1347, Philotheus was consecrated Metropolitan of Heraclea, in Thrace. Becoming a protégé of co-Emperor John VI Kantakouzenos, Bishop Philotheus spent most of his time in Constantinople.

In 1354, Philotheus was appointed Patriarch of Constantinople by John VI. In 1355, after John V Palaiologos obtained the abdication of John VI and forced him into a monastery under the name Joseph Christodoulus, he then forced the deposition of Patr. Philotheus.

In 1364, Callistus stepped aside and helped restore Philotheus to the patriarchal throne. In 1376, Patr. Philotheus was deposed by Emperor Andronikos IV Palaiologos, when the latter ascended to the imperial throne. Philotheus reposed in exile in 1379.

In 1354. the Ottoman Empire gained a foothold in Europe, at Gallipoli, threatening Constantinople from a new side. Threatened anew, John V appealed to the West for help in defending Constantinople against the Turks, proposing, in return, to end the East–West Schism between Constantinople and Rome. Opposed to re-union on political terms, Philotheus was against the efforts by John V to negotiate with Popes Urban V and Gregory XI.

Philotheus pursued an ecclesiastical policy to consolidate under the jurisdiction of the Constantinople patriarchate the Orthodox churches of the Serbians, Russians, and Bulgarians and to assert a primacy over the entire Eastern Church. He actively intervened in the affairs of Russia such that all the ecclesiastical administrative functions were consolidated under the Metropolitan of Kiev, Cyprian, who resided in Moscow.

Philotheus was an advocate of Hesychasm, and aided the cause of the Hesychasts in 1368 by supporting the canonization of Gregory Palamas at a local synod. One notable example of the campaign to enforce the orthodoxy of the Palamist doctrine was the action taken by patriarch Philotheos to crack down on Demetrios and ProchorusCydones. With the support of his younger brother Prochoros, Demetrios Kydones opposed as polytheistic or pantheistic the Palamites and their system of Hesychasm. Applying Aristotelian logic to the Neoplatonic character of Hesychasm, the Kydones brothers accused Palamas of Pantheism or Polytheism, only to be condemned themselves by three successive Palamite synods that also canonized Palamas and Hesychasm. The two brothers had continued to argue forcefully against Palamism even when brought before the patriarch and enjoined to adhere to the orthodox doctrine. Finally, in exasperation, Philotheos convened a synod against the two Cydones in April 1368. However, even this extreme measure failed to effect the submission of Cydones and in the end, Prochorus was excommunicated and suspended from the clergy in perpetuity. The long tome that was prepared for the synod concludes with a decree canonizing Palamas who had died in 1359.[1]

^Jugie, Martin. "The Palamite Controversy". Retrieved 2010-12-28. The reply of Demetrios Cydones reply to the Hesychasts upon his excommunication under Patriarch Philotheos Kokkinos is considered a classic of Catholic polemic against Hesychasm.